cmd/asm/internal/asm: fix comment in ppc64.go

It referred to the wrong architecture.

Fixes #10355.

Change-Id: I5b9d31c9f04f3106b93f94fa68c848b2518b128e
Reviewed-on: https://go-review.googlesource.com/8495Reviewed-by: Dave Cheney <dave@cheney.net>

src/cmd/asm/internal/arch/ppc64.go

@@ -2,9 +2,9 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// This file encapsulates some of the odd characteristics of the ARM
-// instruction set, to minimize its interaction with the core of the
-// assembler.
+// This file encapsulates some of the odd characteristics of the
+// 64-bit PowerPC (PPC64) instruction set, to minimize its interaction
+// with the core of the assembler.
 
 package arch